1. Field of the Invention
The field of this invention lies within the plastic reinforcing art.
2. The Prior Art
The prior art relating to plastic reinforcing materials involves the use of pre-impregnated or coated fabrics or mats of which all or a portion of the individual fibers have been surrounded by the resin. When used for making contoured laminates, such materials offer resistance to bending and stretching to the desired shape of the end product. This is due in part to the resin viscosity between the adjacent fibers restricting relative movement between fibers.
When the resin has low viscosity, the laminating fabric can be formed readily but will not stay in place, thereby pulling loose when other portions of the fabric are handled or by its own weight when placed on vertical surfaces.
When the resin has the proper degree of hard tack to prevent this displacement after being secured in place, the fabric is very difficult to bend and stretch into position. In other words, it tends to have a memory, thus pulling away from sharp contoured areas.
This invention overcomes the foregoing deficiencies of the prior art by creating a combination of fabric or mat and resin that will not spring back after being bent over sharp corners or formed over severe compound contours. The combination consists of a laminating fabric having a discrete uncured resin on both surfaces. The resin is applied in such a manner that it is substantially adhered to only the surface of the fabric or mat by means of the inherent, desired, hard tackiness needed for many types of laminating.
During the bending, the composite acts as a sandwich material having ductile facings of zero yield strength, thereby sandwiching a very pliant core. Such a system has limited spring-back after bending because the tension face or surface has stretched and the compression face has compressed to new lengths.
Some of the resins used require cold storage to prevent deterioration. Even though they are pliable and tacky at room temperature, they are very brittle when cold. In the configuration of this invention they tend to flake away very readily from the surface of the fabric or mat. To prevent this, a very lightweight mat carrier can be incorporated in the resin coatings in the form of a web of strands.
Typical laminating resins are made into sheet form by hot melt coating release film or solvent casting on release films and subsequent drying of the cast sheet. To obtain the desired combination of this invention, the hot melt formed resin sheet is cooled to room temperature and the solution casting is dried to remove all solvent prior to being adhered to the surfaces of the laminating fabric. This assures minimum penetration through the surface fibers to those lying thereunder.
The foregoing product and process enables one to use a fabric that can be configured to compound curves and sharp bends so that the combination fabric and resin will maintain its situ prior to its final molding under heat and pressure. Using the combination of this invention, laminates have been made more easly than with conventionally coated fabrics of one third the weight and thickness with the weave pattern and fabric fiber material being the same. Thus, the labor cost is reduced to one third of what it was. Material cost is also substantially reduced.
Another advantage of this invention is that it is possible to obtain substantially void free laminates after a vacuum molding heat cure cycle. This is due to the fact that the resin is heated to flow through the fabric after a vacuum has been applied to the fabric. Also, conventionally coated fabrics are saturated with resin at ambient pressure in the fabric interstices, causing an entrapment problem.